Patent GB-1416311-A (GB)
Title: Hydraulic power transmission
IPC: F16H61/46
Filed: 1972-04-18
Abstract: 1416311 Hydrostatic transmissions CONVEYANCER Ltd 9 April 1973 [18 April 1972] 17989/72 Heading F2W [Also in Division G3] In a hydrostatic transmission having a variable displacement pump 3, which supplies an hydraulic motor (not shown) and a boost pump 4 which supplies make-up fluid to the main circuit 5, 6 through check valves (not shown) and also supplies pressure to a double acting jack 17 to vary the pump displacement, the pump displacement is set to any one of a number of discrete ratios (three forward and three reverse as shown) by a ratio selector 33 in the form of an electrical switch, which in turn controls an electrical follow-up servo system comprising (in the embodiment shown) a pair of printed circuit patterns 39 and 53 over which brushes can move and a pair of solenoid valves 23 and 24 which connect the pump 4 to one side or the other of the jack through lines 21 and 22 respectively. The jack 17 moves the brushes of the circuit patterns as the piston-rod 16 moves to vary pump displacement. The neutral connection 0 of the selector is connected to the central segment 0 of each pattern through lead 43, the first forward ratio is connected to segment 2, to the right of 0, of both patterns, the second ratio to segments 5 and the third to segments 9, with diodes 50, 51, 52, 54, 55, 56 interconnecting these segments, pointing the opposite way in pattern 39 to that in pattern 53. Similar connections are made for the reverse ratios. Central conductor bars 60, 61 are connected to a selected segment of pattern 39 and bars 62, 63 are connected to a segment of pattern 53, by the brushes. The bars 60, 61 are connected through diodes, a rectifier and a lead 66 to solenoid valve 23, and bars 62, 63 are similarly connected to valve 24, via lead 74. To engage first ratio the ratio control lever 33A is moved to bridge contacts 1 which allows current to pass, assuming main switch 12 and safety switches 37 (handbrake) and 35 (seat) are closed, to segments 2 to the right of segments 0 (for forward). Due to the orientation of the diodes current can pass from segment 2 to segment 0 in the pattern 53 via diode 57 and then through the brush and bars 62, 63 to the valve 24, to keep it open, but cannot pass to valve 23 so this valve closes and vents the right hand side of the jack 17 to move the pump control, which movement also moves the brushes to the right until they bridge the segments 2 and reopen valve 23 to maintain the pump in its adjusted position (non- return valves 26, 27 preventing reverse flow, and so preventing the spring mechanism 20 from returning the jack). Other ratios are selected in similar fashion. A clutch pedal 32 controls a valve 29 which can interconnect the two chambers of jack 17 to allow smooth changes of gear ratio. A relief valve is provided for the boost pump 4. The transmission may be provided with a safety interlock through leads 88, 89 which prevent starting of the engine 1 with the ratio selector 33 in other than neutral, or alternatively this interlock may require the clutch pedal to be depressed. In neutral a green light 82 is illuminated and a red light 85 illuminates otherwise. The ratios selected may be altered by choosing segments of the patterns 39, 53 other than 2, 5 and 9 as shown. In a further embodiment, for the control of the lift and tilt functions of a fork-lift truck, a single acting jack is provided, controlled (for variation of pump displacement in one direction only) by a printed circuit and wiper device similar to that shown (Fig. 2, not shown). A yet further embodiment (Figs. 3, 4, 5 and 6, not shown) has, in place of a jack, an electric motor for variation of pump displacement, and a cam which can operate switches (analogous to the wiper and printed circuit assembly shown) for switching off of the motor on attainment of its desired position. In the embodiment Fig. 2, not shown, means are provided, in the form of a switch (134), to limit the maximum load on an electric motor driving the variable displacement pump, too great a current drawn by the motor operating the switch to affect the solenoid valves such that the pump displacement is limited, or for greater load, reduced to zero.